x86: Remove unused IOMMU_STRESS Kconfig

Last use of IOMMU_STRESS was removed in commit 29b68415e3 ("x86:
amd_iommu: move to drivers/iommu/"). 6 years later the Kconfig entry is
definitely due for removal.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Joerg Roedel <jroedel@suse.de>
Link: https://lkml.kernel.org/r/1516825754-28415-1-git-send-email-clabbe@baylibre.com

arch/x86/Kconfig.debug

@@ -169,14 +169,6 @@ config IOMMU_DEBUG
 	  options. See Documentation/x86/x86_64/boot-options.txt for more
 	  details.
 
-config IOMMU_STRESS
-	bool "Enable IOMMU stress-test mode"
-	---help---
-	  This option disables various optimizations in IOMMU related
-	  code to do real stress testing of the IOMMU code. This option
-	  will cause a performance drop and should only be enabled for
-	  testing.
-
 config IOMMU_LEAK
 	bool "IOMMU leak tracing"
 	depends on IOMMU_DEBUG && DMA_API_DEBUG